FC lower than expected

[quantumgirl]

Newbie here I have an 8000 gallon liner pool. We just refilled it because I wasn’t able to get any kind of chlorine reading even after dumping 5 lb of shock in at the time. CYA was at 130. I’m in Houston.

So I bought the test kit. Made the switch to liquid chlorine. Clean pool with fresh water. Adjusted pH. Added some CYA. Last night my FC was at 0 (24 hrs after refill with tap water which initially had some chlorine in it). So I added 1 gallon of liquid chlorine from Leslie’s (10-12.5% concentration). According to pool maths, it should have gotten me up to an FC of 12. I added the chlorine after sunset.
This morning right at sunrise I got those numbers:

Ph-7.5
TA- 80
CYA- maybe right at 30 or a little less
FC- 5
CC-0.5

Question: how come FC didn’t get all the way to 12 with a whole gallon of liquid chlorine? Is this normal? I can’t afford to add an entire bottle every day. So if the sun wasn’t out, and it’s brand new water that shouldn’t be contaminated, where did the chlorine go?

 

[mknauss]

Chlorine was old unless your pool volume is wrong.

 

[quantumgirl]

Chlorine was old unless your pool volume is wrong.

Well according to Leslie’s it had just come in on the truck the day prior

 

[tim5055]

Because you probably still have residual algae in the water from when you couldn't get any kind of a chlorine reading.

How much CYA did you add? What was your target with the amount you added?

Tonight, lets add about half a bottl of that 10/12% chlorine. Let the pool circulate and after the sun sets get a chlorine reading. Tomorrow before the sun is up lets get another one and see what exact loss is.

 

[tim5055]

Well according to Leslie’s it had just come in on the truck the day prior

Look on the side of the bottle. The manufacture date will be in small black letters/numbers.

Something like 220125XXXXX That would be manufacturered on the 125th day of 2022. Even fresh, if they stored it out in the heat it degrades quickly.

 

[quantumgirl]

You’re saying I still have residual algea. Where would that be hiding if I drained all the water and scrubbed the liner? Should I have replaced the sand in the filter?
I added about 3 lb of CYA, trying to get up to 40. I assume not all of it is in circulation yet. I poured it into the skimmer basket per instructions on the bag. That was before I read about the sock method

As far as the chlorine goes, I cannot find any kind of printed number on the bottle brand is blue whale. It was a 4 pack so maybe the number would have been on the cardboard box which I trashed already

 

[Mdragger88]

2 things -
Do an Overnight Chlorine Loss Test to rule out algae as Tim mentioned.
& check fc 30 minutes after dosing to be sure of liquid chlorine potency so u can adjust accordingly with this batch.
Also some questions-
What kind of pool is this-
Above ground or in ground?
Are there steps/ ladder?
Was there obvious algae or cloudiness before the water exchange?

 

[quantumgirl]

2 things -
Do an Overnight Chlorine Loss Test to rule out algae as Tim mentioned.
& check fc 30 minutes after dosing to be sure of liquid chlorine potency so u can adjust accordingly with this batch.
Also some questions-
What kind of pool is this-
Above ground or in ground?
Are there steps/ ladder?
Was there obvious algae or cloudiness before the water exchange?

It’s a vinyl liner pool that’s sitting 2/3 into the ground. Aluminum ladder.
So we were on vacation for 3 weeks and didn’t have anybody to deal with the pool while we were gone. Obviously it was green when we got back home. Shocking the water did clear it up. I just wasn’t able to get and maintain any kind of chlorine reading even after adding obscene amounts of shock. So we figured we’d just completely drain it and start fresh, since CYA was way too high anyway.

I’ll do the overnight test tonight.

 

[tim5055]

Well, don't backflush the filter for a few days at least. Dumping CYA in the skimmer can leave it trapped in a sand filter as it slowly dissolves. If you backwash, it goes away.

Algae can hide lots of places. You didn't get all the water, nooks & crannys lights and ladders will hold it.

3 pounds of CYA would raise the level by 45 in y our 8,000 gallon pool

 

[IceShadow]

Best to do what was recommended - after sunset, add the chlorine, circulate for 15-30 minutes, then test to see what it increased to. Then, test again in the morning before sunrise to see if it's dropped from the reading at sunset.

This will both let you check to make sure that the chlorine is still good (if the FC raised up to the expected levels) and see if there's hidden algae somewhere (seeing if it drops overnight).

 

[quantumgirl]

Just got back from Leslie’s. So I assume I’m good with the CYA. FC seems to be dropping rather rapidly if it’s even possible to compare their test with my reading. I got an FC of 5 just 5 hours prior to this.
Calcium hardness and Phosphate reading is nothing really to worry about, is it? They were trying to sell me phosphate remover

 

[IceShadow]

The Leslie's test isn't worth the paper you have it printed on.

You have a good kit. Use and trust it. Don't bother with a Leslie's test.

CH isn't needed in a vinyl pool, except maybe if a heater is involved. Phosphates aren't important at all, except as insurance if your FC additions lapse and get a bit low (and anecdotally for some SWCGs).

EDIT: I take it back, the test does have one redeeming quality - the metals test. Good to know you don't have metals. (Home metals testing is so expensive that we have to use pool store testing for that.)

 

[mknauss]

Your signature shows a TFPRO test kit. Why are you going to a pool store? We do not provide guidance based on pool store testing.

 

[quantumgirl]

Your signature shows a TFPRO test kit. Why are you going to a pool store? We do not provide guidance based on pool store testing.

Yeah I just wanted some reassurance on my CYA reading. I found that test to be somewhat tricky to read. I’ll try to stay away from the enemy from now on

 

[IceShadow]

Yeah I just wanted some reassurance on my CYA reading. I found that test to be somewhat tricky to read. I’ll try to stay away from the enemy from now on

If you'd like reassurance, I would recommend getting the 50ppm CYA standard solution and test that. I've heard it can help with developing an "eye" for the CYA test.

 

[quantumgirl]

I did the overnight test. FC before adding chlorine was 2.5.
I added one gallon liquid chlorine (12.5%) after dark. 30 minutes later FC was at 14. Tested again this morning right before sunrise and FC is at 13.
So can I assume that whatever the problem was has just fixed itself?

 

[PoolStored]

I did the overnight test. FC before adding chlorine was 2.5.
I added one gallon liquid chlorine (12.5%) after dark. 30 minutes later FC was at 14. Tested again this morning right before sunrise and FC is at 13.
So can I assume that whatever the problem was has just fixed itself?

So you pass OCLT. Congrats!

Your pool water FC (Free Chlorine) will drop during the day as the sun consumes FC and killing stuff (bacteria/algae/organics) will use FC. You need to test and replace FC with liquid chlorine daily. Never let your FC go below minimums on this chart. -->FC/CYA Levels

If your CYA is 40, then your minimum is 3 and your target is 5-7. If your pool consumes 4ppm FC per day, you want to add enough chlorine to get to 8 on the chart because at the end of the day it will be 4 (you want to stay above minimum). If my pool even sniffs minimums it will cloud. You will "learn" how much FC your pool needs everyday through your testing.

Do a test today! If you are at 13 this morning, don't add anything to the pool, test after sunset. That will tell you what you how much FC you are consuming daily. Take that number, add minimum for your FC and then add 1 for safety. That is your target for when you add LC.

Example: CYA 40, your minimum is 3. You go from 13 this morning to 10 tonight your FC usage was 3. Your target FC would be:
3 + 3 + 1 = 7. If you replenish to 7, you will not go below 4 with a usage of 3.

Does that make sense?

 

[quantumgirl]

So you pass OCLT. Congrats!

Your pool water FC (Free Chlorine) will drop during the day as the sun consumes FC and killing stuff (bacteria/algae/organics) will use FC. You need to test and replace FC with liquid chlorine daily. Never let your FC go below minimums on this chart. -->FC/CYA Levels

If your CYA is 40, then your minimum is 3 and your target is 5-7. If your pool consumes 4ppm FC per day, you want to add enough chlorine to get to 8 on the chart because at the end of the day it will be 4 (you want to stay above minimum). If my pool even sniffs minimums it will cloud. You will "learn" how much FC your pool needs everyday through your testing.

Do a test today! If you are at 13 this morning, don't add anything to the pool, test after sunset. That will tell you what you how much FC you are consuming daily. Take that number, add minimum for your FC and then add 1 for safety. That is your target for when you add LC.

Example: CYA 40, your minimum is 3. You go from 13 this morning to 10 tonight your FC usage was 3. Your target FC would be:
3 + 3 + 1 = 7. If you replenish to 7, you will not go below 4 with a usage of 3.

Does that make sense?

Perfect sense. Thank you for the detailed explanation!

 

[tim5055]

So can I assume that whatever the problem was has just fixed itself?

The chlorine was just doing it's job. Something was in the water and it eliminated it.

@PoolStored gave you a great rundown of things moving forward.

Continued testing is the key.

Use the FAS/DPD (powder) test every day until you get a feel for your pool and it's appetite for chlorine. At that point you can cut the FAS/DPD test down to a few days a week spaced out and use the OTO test (the drop color match) the other days. The OTO test is not as accurate, but you are just using it to make sure you are in the ballpark those days you don't do teh FAS/DPD.

For me I did a pH test every day for a while. Once I figured my pH change I cut that back to a couple/few days a week.

TA & CH i only test monthly unless my pH rise changes and I'll look at TA then

CYA I only test a few times a swim season.

 

[quantumgirl]

So yesterday throughout the day I lost 6.5ppm which seemed really high. So I did another overnight test and lost 4 ppm this time
I guess that means I need to do the whole slam thing? I hate this darn pool